DNA gyrase, which is involved in DNA replication, integrative recombination of phage lambda DNA, DNA repair, and transcription, was purified from E. coli, T4-infected E. coli, or B. subtilis. The enzyme consists of the nalidixic acid (oxolinic acid) resistance and novobiocin (coumermycin A1) resistance gene products. In this year, the reaction mechanism of DNA gyrase has been extensively studied in order to determine the role of this enzyme in many important processes in cells. The mechanisms of inhibitory action of novobiocin and nalidixic acid have also been investigated.